Electro-optical measuring instruments

ABSTRACT

The present invention relates to an optical routing module (2) suitable for use in a light microscope (1) for sample inspection simultaneously with a primary light source (10) and a secondary light source (7) of different wavelength. The module (2) comprises a housing mounting first and second polarizing beam splitters PBS1, PBS2 along a primary light beam pathway through the module and having secondary light beam inlet and outlet means (5,6) opposite different ones of the polarizing beam splitters PBS1, PBS2, which have a narrow predetermined operating wavelength range, defined between s- and p-plane transitional wavelengths, which substantially excludes the primary light source wave length band and such that at least one polarizing plane component of each of the secondary light source and a secondary light output from the sample is subjected to a different one of transmission and reflection from that to which the primary light source is subjected at each of the first and second polarizing beam splitters PBS1, PBS2, which are further formed and arranged for defining a secondary light beam pathway from the inlet (5) to the outlet (6) so that the secondary light beam pathway is brought substantially into alignment with an outward leg of said primary beam pathway upstream of the sample by said first polarizing beam splitter PBS1 and is separated back out from a return leg of said primary light beam pathway downstream of the sample by said second polarizing beam splitter PBS2 whereby in use of the module (2) in a light microscope (1), the area of incidence of the secondary light beam with the sample may be monitored via the primary light beam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electro-optical measuring instrumentsand to optical routeing modules and gating devices suitable for usetherein.

2. Discussion of Prior Art

With ever-increasing miniaturization of electronic circuits there is aneed for increasingly sophisticated analytical techniques operating atever higher resolutions. One such technique involves the use ofPhotoluminescence Lifetime Spectrometers (PLS) for measurement ofphotoluminescence in semi-conductors especially those of compounds suchas Gallium Arsenide (Ga As) which are more susceptible to the incidenceof structural discontinuities due to local crystallisation defects, suchdefects being detectable by variations in the photoluminescent outputthereat.

In more detail, photoluminescence is the emission of a photon uponrecombination of an electron-hole pair generated by photoexcitation of asemiconductor. The term photoluminescence is normally used to describethis mechanism in solids, whereas the term fluorescence describesanalogous processes in atoms and molecules.

The photoluminescence intensity is related to the number of excitedelectron-hole pairs, or, in other words, the excess carrier densities.These carriers eventually decay, the carrier lifetime being determinedby the rates of decay via various (electron-hole pair) recombinationmechanisms. Photoluminescence is the result of a radiative recombinationprocess and can be observed externally, due to photons leaving thesample surface. The carrier lifetime can be determined from thephotoluminescence lifetime by straightforward interpretation.

The carrier lifetime and the closely related carrier diffusion lengthare the most important parameters characterising the electronicproperties of a semiconductor In order to predict and explainsemiconductor device performance, these two parameters have to be known.

In gallium arsenide (GaAs), the technologically second most importantsemiconductor after silicon (Si), the carrier lifetime is of the orderof 10 ps to 1 μs (10-¹¹ -10⁻⁶ s), depending on the influence(presence/absence) of localized non-radiative recombination centers. Itis hence obvious that spatial fluctuations of the carrier lifetime canoccur on a scale comparable to the carrier diffusion length, typicallybelow 1 to 10 μm. In fact, strong inhomogeneities in the carrierlifetime have been experimentally observed by averaging over about 100μm, i.e. many times the diffusion length in GaAs, but closerinvestigation has so far been impossible due to lack of an experimentaltechnique.

It is known that integration density of gates on GaAs chips is stillvery low compared with Si, because of the inhomogeneity of GaAs wafers.hence there is great commercial interest in experimental methods capableof measuring the carrier lifetime of such materials with high temporaland high spatial resolution

Time-correlated single photon counting (TCSPC) is an experimentaltechnique for measuring the dynamic behaviour of excited electronicstates in atoms, molecules and solids. At present, this technique iswidely applied in photochemistry and photobiology with many commercialsystems for fluorescence decay measurements already on the market.However, due to lack of fast single photon detectors with highsensitivity in the near infrared, its application to semiconductors hasso far been very limited.

It is important to realise, that the TCSPC technique is several ordersof magnitude more sensitive than any other experimental technique formeasuring time-resolved photoluminescence. This permits the measurementof photoluminescence with very high spatial resolution. With othertechniques a large photoexcitation density is required in order toobtain a sufficiently large signal from a small sample area. However,the upper limit for the tolerable excitation density is often the dopingconcentration. In other cases, e.g. in testing a laser diode resonatorstructure of typically 50×3×1 μm³ size, the excitation density may haveto be even lower in order to stay below the onset of induced emission.

Taking into account only the inevitable losses occurring in the sampleitself, internal quantum efficiency, geometric factors, surfacereflection etc., the signal intensity available from a sample area ofonly a few μm diameter can be as low as a few photons per excitationpulse. Even with a highly efficient spectrometer, this signal intensityis reduced even further by spectral discrimination etc. before beingdetected.

Previously known apparatus is unable to separate out and extract suchvery low intensity output signals at the high spatial resolutionsrequired to pin-point any microscopic defects that may be present on thesemi-conductor surface or in the body volume probed by the photoexcitedcarriers. In addition there is the major problem, once the occurrence ofindividual luminescence photons has been accurately detected, ofmeasuring the elapsed time between the detected photons and theassociated excitation pulses which originally gave rise to them, giventhe extremely large numbers of excitations pulses for which no outputphotons are detected, as well as the very high excitation pulsefrequencies which are used in practice in these studies.

SUMMARY OF THE INVENTION

It is an object of the present invention to avoid or minimize one ormore of the above disadvantages.

The present inventor has now found that by using an optical routeingmodule of the invention in a light microscope with infinity correctedoptics, a secondary light beam pathway can be routed into and out of aprimary light beam pathway of the microscope with very high spatial andsignal resolution allowing very precise monitoring of the area ofincidence of the excitation signal with the sample and extraction ofphotoluminescent photons output from the primary beam pathway verysubstantially free of excitation pulse and primary light beam signals.

Thus in one aspect the present invention provides an optical routeingmodule device suitable for use in a light microscope for sampleinspection simultaneously with a primary light source and a secondarylight source of different wavelength to said primary light source, whichdevice comprises a housing mounting first and second polarising beamsplitter means along a primary light beam pathway through said deviceand having secondary light beam inlet and outlet means disposed oppositedifferent ones of said first and second polarizing beam splitter means,said first and second polarising beam splitter means each having anarrow predetermined operating wavelength range, defined between s-planeand p-plane transitional wavelengths, which substantially excludes theprimary light source wavelength band and is substantially above or belowthe wavelength band of said primary light source and such that thewavelength band of each of the secondary light source and a secondarylight output from the sample responsive to incidence of said secondarylight source on a said sample in use of the device is substantiallybelow the s-plane transitional wavelength or above the p-planetransitional wavelength, of a respective one of said first and secondpolarising beam splitter means, respectively whereby, in use of thedevice, at least one polarising plane component of each of the secondarylight source and said secondary light output is subjected to a differentone of transmission and reflection from that to which the primary lightsource is subjected at each of the first and second polarising beamsplitter means, said first and second polarising beam splitter meansfurther being formed and arranged for defining a secondary light beampathway from said inlet to said outlet so that the secondary light beampathway is brought substantially into alignment with an outward leg ofsaid primary beam pathway upstream of the sample by said polarising beamsplitter means and is separated back out from a return leg of saidprimary light beam pathway downstream of the sample by said secondpolarising beam splitter means whereby in use of the device in a lightmicroscope, the area of incidence of the secondary light beam with thesample may be monitored via that small portion of the primary light beamreflected from the sample and leaked through the pair of polarizingbeamsplitters into the primary light beam pathway.

With such a routeing module, the primary and secondary light beampathways are brought together and separated out again at the first andsecond polarising beam splitters, respectively. In addition, by using amodule arrangement wherein a defined polarisation state, i.e s- orp-plane polarization or circular/olliptic polarization, of the secondarylight source beam incident upon the sample is altered, e g. byphotoluminescence or the elctrooptic effect, the modified secondary beamsignal may be very substantially separated from unmodified reflectedsecondary beam light source signals, even where there is little or nodifference in wavelength between the two. Thus the module maximisessignal resolution by using polarization discrimination together withspectral discrimination.

It will be appreciated that various different forms of module may beused with diverse spectral characteristics according to the differenttypes of primary and secondary light sources required to be used for anyparticular sample and/or type of investigation. It may be noted herethat modules of the invention may be used for diverse investigationssuch as fluorecence in microscopic biological samples such as individualplant or animal cells and in electro-optic sampling where voltagecharges in microcircuits are monitored through the changes inpolarisation they produce in an electrooptically active material such ase.g. GaAs itself.

Thus for example there may be provided different preferred forms ofmodule of the invention depending upon whether the secondary lightsource has a longer or shorter wavelength than the primary light source,the polarising beam splitters having different transitional wavelengthsand, if required, being disposed in different physical arrangementsaccording to which (primary or secondary) beam pathway is to bereflected or transmitted at each of the polarising beam splitters.Naturally additional optical elements may be used as required includingfor example, polarising plane rotation means such as half-wave plates tochange the plane of polarisation of a plane-polarised beam, planereflecting means such as mirrors and/or plain prisms for changing thedirection of a beam, and filters e g. for selective spectraltransmission. It will also be appreciated that where there is asignificant difference in wavelength between the secondary light sourceand the secondary light output, the operating wavelength ranges (betweenthe s-plane and p-plane transition curves) of the first and secondpolarising beam splitters, may be partly offset i e. spectrally shiftedrelative to each other.

In a further aspect the present invention provides an opticalspectometer device suitable for use in photoluminescence inspection ofmicroscopic areas of microstructures which device comprises a lightmicroscope having infinity corrected optics with a primary light beampathway from a primary light source to a primary beam image outputmeans, via a sample stage which primary light beam pathway has asubstantially common portion extending towards and away from said samplestage, charaterised in that there is provided an optical routeing moduledevice of the invention along said primary light beam pathway commonportion, together with a secondary light beam source and a secondarylight beam detector means coupled to respective ones of said secondarylight beam inlet and outlet means of said module, whereby the positionof incidence of said secondary light beam on said sample may beprecisely directed by montoring of said primary beam image output means.

Conveniently, for photoluminescence studies especially the secondarylight beam source comprises a laser source. With the high resolutionsand sensitivity provided by the present invention it is moreoverfeasible to employ a pulsed diode laser source despite the relativelylow power of such means. This in turn is particularly advantageous sinceit avoids the need for the relatively cumbersome and delicate benchmounted mode-locked and/or cavity dumped solid state or gas, lasers,thereby making the spectrometer significantly simpler, more portable andeconomical than was previously possible. Morevover the use of opticalfibre coupling means makes the apparatus significantly more flexible andeasier to align.

Any suitable optical processing means may be coupled to the secondarylight beam outlet of the module including one or more of a solid statedetector, a fibre optic monochromator, and wavelength-division-multiplexer. Most desirably there is used a single photon avalanchediode detector, attached to the outlet directly, fiberoptically, or viaspectral discriminating means.

In order to maximize the temporal resolution of the spectrometer, thepresent invention provides in yet another aspect an anti-coincidencegating means comprising first and second discriminator pulse processingmeans for receiving respective ones of an excitation signal inputcomprising a series of pulses corresponding to the laser sourcesecondary light beam pulses and a detector signal output comprising aseries of pulses corresponding to secondary light output pulses inducedby incidence of said laser source pulses with the sample in use of thedevice, said first pulse processing means being formed and arranged forproviding first and second outputs comprising a first series of pulsescorresponding to the laser source pulses substantially free ofinterference, said second pulse processing means being formed andarranged for providing a first output comprising a second series ofpulses corresponding to secondary light output pulses substantially freeof interference and a second output comprising a second series of gatepulses for respective ones of said secondary light output pulsestemporally extended to not longer than the period between successivesaid laser pulses; a third pulse processing means formed and arrangedfor receiving the second output of said first pulse processing means andsaid gate pulses and providing a modified output corresponding to saidfirst series of pulses from which have been removed, by said gatepulses, those pulses associated with said secondary light output pulsesthereby providing an anti-coincidence series of pulses; and a fourthpulse processing means formed and arranged for receiving said firstoutput comprising a said first series of pulses and said modified outputcomprising said anti-coincidence series of pulses, and combining them soas to provide a reduced output comprising pulses corresponding to onlythose of said laser pulses for which a secondary light output pulse hasbeen received, said gating means further including pulse delay meansformed and arranged for providing said first output of secondary lightoutput pulses in the same temporal relationship to said reduced outputof pulses corresponding to corresponding laser pulses, as in theexcitation signal input and detector signal output whereby monitoring ofsaid temporal relation is substantially free of interfernce fromexcitation signals for which no detector signal output is received.

The anti-coincidence gating means of the invention is of a particularlyconvenient and economic form.

BRIEF DESCRIPTION OF THE DRAWINGS

Further preferred features and advantages of the invention will appearfrom the following detailed description given by way of example of somepreferred embodiments illustrated with reference to the accompanyingdrawings in which:

FIG. 1 is a general view of a photoluminescence lifetime spectrometer ofthe invention with its associated signal processing equipment;

FIG. 2 is a schematic perspective view showing the principal opticalcomponents of the optical routeing module and associated microscopeobjective of the spectrometer of FIG. 1;

FIG. 3 is a sectional view of a generally similar module showing itsrelation to the other parts of the microscope of the spectrometer;

FIGS. 4A and 4B are schematic sectional elevations of two opticalrouteing modules for use in application with different spectralrequirements, shown together with the spectral transmissioncharacteristics of the polarising beam splitter of the respectivemodules;

FIGS. 5A and 5B are block diagrams illustrating the anti-coincidencegating means of the invention and its mode of operation;

FIGS. 6A and 6B are graphs showing a typical photoluminescence decaycurve obtained with a photoluminescence lifetime spectrometer of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a photoluminescence lifetime spectrometer (PLS) comprisinga light microscope 1 which has infinity corrected optics and an opticalrouteing module 2 of the invention, signal processing apparatus 3, forprocessing the secondary beam output of module 2 and its temporalrelation the secondary beam input to module 3, and data analysis meansfor converting the output signals from the signal processing apparatus 3into a useful form.

In more detail the microscope 1 has infinity corrected optics, i e apoint source in the object plane forms a parallel beam within themicroscope optical column. This allows variation in the optical pathlength, e.g. by extending the length of the microscope tube andinserting a number of plan-parallel optical components into the beampath, without affecting the image quality. Several commercial microscopesystems now have this feature. In the present embodiment an Olympusmetallurgical series BH2 microscope was chosen for its convenientmodular design, which makes it very easy to split the optical column inthe parallel beam region of the microscope and insert the opticalrouteing module 2 (ORM). The ORM 2 has inlet and outlet ports 5, 6 forcoupling a secondary light pulsed laser excitation source 7 into themicroscope 1 and suitable detector 8 for secondary lightphotoluminescence signal photons out of the microscope 1. Optional ports7 are provided to pick up a synchronisation signal off the pulsed laserlight and pick up scattered excitation light for measuring theinstrumental response profile.

In other respects the microscope 1 is of generally conventional formhaving a primary light source e.g. visible or N1R light illuminator 10an optional binocular eyepiece 11, and a CCD camera 12 coupled to amonitor 13. It will be appreciated that the ORM 2 requires to providefor proper routeing of each of the following:

1 a primary light beam comprising visible light from the microscope'silluminator 10 to a sample disposed on the sample stage 14 of themicroscope 1 and thence back to the CCD camera 12 and binocular eyepiece11 for inspection of the sample through the optical microscope 1;

2 a secondary beam input comprising visible/near-infrared pulsed laserlight from the laser excitation source 7 to the sample for excitation ofphotoluminescence and from the sample surface scattered and reflectedback off into the CCD camera 12 and binocular 11 for alignment, laserbeam focusing and positioning on the sample; and

3 near-infrared photoluminescence photons from the sample to thedetector 8 for time-correlated single photon counting.

If maximum spatial resolution and/or maximum signal collectionefficiency of the spectrometer is desired, one has to use optics withhigh numerical aperture (N.A.) for focusing the excitation light on tothe sample and collimating the hemispherically emitted photoluminescencephotons. The working distance then reduces to only a few millimeters,which automatically excludes conventional geometries with different axesfor excitation and detection such as the well-known L- and T-geometryused in fluorometry.

As a result, excitation light and photoluminescence have to be handledthrough the same microscope objective and signal separation becomes aproblem.

The targets to be met by the ORM design are as follows:

i) suppression of excitation light in the detection channel

ii) high photoluminescence signal collection efficiency

iii) diffraction limited performance

iv) no degradation of optical microscope performance

v) modular design

vi) definition of simple interfaces for all optical ports

vii) low cost

All optical signals into or out of the spectrometer can be handledthrough single-(and multi-mode) optical fibres. This is possible becauseof the low optical power levels required in the PLS and the ability ofthis kind of system, due to its highly linear instrumental response, tocorrect for the linear dispersion of short optical pulses in the fiberof convolution analysis. Detaching the detector 8 from the ORM isoptional when using filters instead of a monochromator for spectralselection. It is, however, a very attractive option, as it enhancesmodularity and has some very specific advantages when used in connectionwith Single Photon Avalanche Diode (SPAD) detectors in preferred formsof the invention. The use of high precision fibre-optic connectors forlinking up the ORM 2 with the sources and detectors mounted in aninstrumentation rack and housing the signal processing apparatus 3 or,alternatively, any other external laser source, monochromator ordetector, defines a simple, stand optical interface and substantiallyeliminates alignment problems. All high-speed Nuclear InstrumentationMethod (NIM) electronics as well as active secondary beam optical unitscan be conveniently fitted into such an instrumentation rack. Thiseliminates the need for transmitting any high speed electronic signalsalong lossy, inherently noisy coaxial lines outside the instrumentationrack. The detachable instrumentation rack and the data processingapparatus can then be placed at a convenient distance from themicroscope 1, e.g. when the latter is to be used in a cleanroomenvironment.

FIG. 2 shows the principal parts of a First optical routeing module ofthe invention together with the reflecting objective 01 of a microscopein which it is being used.

Linearly polarised pulsed laser light is coupled into the ORM by meansof polarisation preserving single-mode optical fibre FO1. Thepolarisation axis is perpendicular to the plane defined by themicroscope axis and the direction of propagation of the incoming laserlight. The optical fibre/cable is terminated with a polarisationpreserving single-mode fibre-optic connector which plugs into a matingadapter of the ORM housing.

A precision lens L1 with very short focal length is fitted into the ORMside of the mating adapter. The divergent beam from the fibre end iscollimated by this lens for form a parallel beam with approximately 0.9mm beam waist diameter.

Precise alignment and a suitable beam steering mechanism is crucial forachieving high spatial resolution with the PLS spectrometer. It is herewhere the use of optical fibres is most appreciated. As mentionedbefore, the laser beam exiting the optical fibre is collimated by theprecision lens L1 fitted into the mating adapter. The axial distancebetween the fibre end and the lens is initially adjusted by monitoringthe laser spot size produced on the sample, through the microscope.

Manipulation of the excitation laser beam along 4-axes with respect tothe ORM is achieved by mounting the mating adapter on a small roll-tiltand x-y translational stage. by simple adjustment of the position andorientation of the mating adapter, the collimated laser beam isinitially aligned to be parallel to, but with its axis displaced severalmillimeters from the microscope's optical axis, as for the reasonsmentioned below. The optical source can then be disconnected andreconnected to the spectrometer without loss of alignment. this isconsidered very important in practice, as it eliminates tediousalignment after transport, allows simple insertion/removal of differentORMs and greatly enhances overall system flexibility.

As may also been seen in FIG. 3, which is a schematic sectional view ofa generally similar ORM shown incorporated in a light microscope of thegeneral type shown in FIG. 1 (this embodiment also including a half-waveplate λ/2 for changing the plane of polarisation of the excitation beamand various optically plane elements G1-G4 such as filters), thecollimated beam is bent 90° down the microscope column into alignmentwith the primary light beam by a first polarising beamsplitter cubePBS1. Following PBS1, which couples the laser light from the excitationchannel into the microscope, a second polarising beamsplitter cube PBS2,couples the photoluminescence signal from the sample out of themicroscope and into the detection channel.

Polarising beamsplitter cubes were chosen rather than any other type ofbeamsplitter because these are optimised for operation within a smallwavelength range, typically between 0.9 and 1.1 times the designwavelength. Within this range, the propagation axis of light polarisedin the s-plane (with respect to the PBS) is bent by 90°, while lightpolarised in the p-plane is transmitted. Light of shorter wavelengththan 0.9 times the design wavelength is transmitted irrespective of itspolarisation state, light of longer wavelength than 1.1 times the designwavelength is reflected irrespective of its polarisation state. Byvirtue of this property, a pair of crossed polarising beamsplitter cubesdo not prevent light from passing through their common optical axis,except within the small wavelength range of operation around the designwavelength.

If two distinct wavelength ranges are used for optical inspection of asample and excitation/detection of photoluminescence, the addition ofthe ORM to an optical microscope will not affect the microscope'sperformance. Thus for example as illustrated in FIG. 4Aphotoluminescence can be excited e.g. at 780 nm EXC and detected at790-950 nm DET, which is ideal e.g. for investigating the importantsubstrate materials gallium arsenide (GaAs) and indium phosphide (InP).Visible light in the range 400-700 nm passes through the crossedpolarizing beamsplitter cubes unhindered, except for rather strongreflection of that part of the visible spectrum which is far off thedesign wavelength of the anti-reflection coating applied to selectedsurfaces of the polarizing beamsplitter cubes. In order to obtain bestcontrast along the INSP pathway whilst retaining a large field of viewfor inspection (i.e. reflections have to be reduced other than byspatial filtering along the INSP beam pathway), the wavelength range ofthe visible spectrum used for inspection may be deliberately limited to575-675 nm as shown in FIGS. 4A, 4B, i.e. close to the operatingwavelength of the polarizing beamsplitter cubes. In more detail, it maybe seen that the first polarising beamsplitter PBS1 has an operatingwaveband in the region from 720 nm (boundary between transmission(shorter wavelength side to left of curve) and reflection (longerwavelength side to right of curve) for s-plane polarised light (solidcurve)) to 880 nm boundary between transmission and reflection forp-plane polarised light (dashed curve). The operating band of PBS2 maybe seen to be offset relative to that of PBS1 to the range 690 to 850nm. Thus for each PBS, substantially all light of shorter wavelengththan the operating band is transmitted and that of longer wavelength issubstantially all reflected whereas within the range only p-planepolarised light is tramsitted and only s-plane polarised light isreflected.

Thus in the arrangement shown in FIG. 4A on s-plane polarised excitationsource secondary beam is reflected at PBS1 into the primary beam pathwaydown through the half-wave plate λ/2 where it is rotated by 90° intop-plane w.r.t. PBS2. The p-plane polarised excitation source secondarybeam is then transmitted down through PBS2 onto the sample. Excitationlight EXC (s- or p-plane w.r.t. sample surface) specularly reflected offthe sample surface is again transmitted back through PBS2, as also isthe p-plane polarised component of the randomly polarisedphotoluminescence emitted from the sample in response to incidence ofexcitation light thereon. The s-plane polarised component of thephotoluminescence is however reflected at PBS2 and thereby separated outfrom the return leg of the primary light beam pathway thereat and thenpasses on to the detector.

In the embodiment of FIG. 2 it will be noted that the excitation anddetection wavelengths EXC, DET are substantially shorter than theprimary light beam wavelengths INSP and the operating wavebands of PBS1and PBS2 correspondingly shifted to the shorter wavelength and of thespectrum so that in this case all the primary light beam is reflected atPBS1 and PBS2 rather than transmitted there through. This calls for aslightly different arrangement of PBS1 and PBS2 with the ORM includingan additional polarising beamsplitter PBSA acting simply as two planereflectors for each of the primary light beam INSP and the secondarylight excitation source EXC. The latter is directed down through ahalfwave plate (λ/2)1 as before to change the polarisation plane of EXCfrom s- to p- before passing PBS2 and impinging upon the sample. Asbefore the specularly reflected excitation light passes back throughPBS1 as does also the p-plane polarised component of thephotoluminescence. The s-plane polarised component of thephotoluminescence DET and the primary light beam INSP are both reflectedat PBS1 and then passes through a second haf-wave plate (λ/2)2 where thes-plane component of DET is rotated 90° and changed to p- relative toPBS2. Both primary and secondary light beams then continue on to PBS2where the primary light beam INSP is again reflected to pass backthrough a first plane spacer SP1, off a plane reflector prism P4 througha second plane spacer S2, to be once more reflected at the additionalpolarising beam splitter PBSA. the p-plane polarised secondary lightbeam DET is however transmitted through pBS2, and is thereby separatedout from the primary light beam pathway thereat.

Naturally other arrangements based on similar optical principles arealso possible for other circumstances e.g. where the excitationwavelength EXC is shorter than for the primary light beam waveband INSPwhilst the secondary light output wavelength band DET is longer than theprimary light beam waveband INSP. Polarising beam splitter cubes arecommercially available across substantially the whole of the visible andnear infrared spectrum so that ORMs according to the invention may bereadily provided for a wide range of specific applications.

As noted above a reflecting objective (two mirror surfaces) 01 ispreferably used instead of a conventional refractive microscopeobjective (many lenses), because it has a large working distance with nounwanted reflections or chromatic aberration.

In contrast, conventional refractive microscope objectives, optimisedand anti-reflection (AR) coated for the wavelength range of visiblelight, would produce unwanted backscatting signals when used in the nearinfrared and suffer from chromatic aberration. In picosecond pulseexperiments, a narrowband AR-coating optimised for the detectionwavelength would be required to suppress multiple reflections, whichwould broaden the pulse width or produce spurious peaks in the decaycurve.

In addition, narrowband coatings perform very badly outside the verysmall wavelength range around their design wavelength, such as at theexcitation wavelength or at the shorter wavelengths used for opticalinspection in the PLS.

Reflective objectives do not have these drawbacks. A characteristicfeature of these reflecting objectives is the central obstruction, whichmeans that light travelling down the microscope column on or near to theoptical axis is reflected back into the microscope instead of beingpassed through the objective. Because of this, the excitation laser beamis not expanded to the full objective aperture, but only to a rathersmall beam width, such that it is sent through the reflecting objectivebetween the central obstruction and the limit of the aperture withoutsuffering back-reflection.

The converging laser beam thus forms a narrow light cone incident uponthe sample surface at an angle. Reflection of excitation Light off thesample surface is undesirable and can be minimised by passing the laserbeam through the reflecting objective such that the beam is polarised inthe p-plane with respect to the sample surface. For very high N.A.objectives and/or low refractive index samples, the angle of incidenceof the excitation laser beam may approach the Brewster angle, at whichno light is reflected off the sample surface at all.

Photoluminescence is emitted from the sample istropically, i.e.hemispherically and randomly i.e. unpolarised. In order to achieve highcollection efficiency, a reflecting objective with high N.A. (e.g.N.A.=0.65) is desirably used. The polarising beamsplitter cube PBS2couples out only the photoluminescence component polarisedperpendicularly to the excitation laser beam, i.e theoretically 50% ofthe total unpolarised signal.

In practice, polarising beamsplitter cubes suppress the unwantedpolarisation component with respect to the signal by only about 99%,therefore the photoluminescence signal is desirably further purified bypassing it through a further polarisation selective component (see FIG.2). Use of a polarising prism or a further polarising beamsplitter cube(PBS3) results in discrimination of the photoluminescence signal withrespect to back-scattered excitation light by a factor of at least 10⁴.

The photoluminescence signal then desirably is further discriminatedspectrally, which can be achieved either with narrowband interferenceand/or long-pass filters or with a suitable monochromator.

However, although spectral selection discriminates against scatteredexcitation light and possibly ambient light leaking in, it cannotdiscriminate against multipy reflected photoluminescence photons at thedetection wavelength. As the reflectivity of even AR-coated opticalsurfaces is of the order of 0.5-1%, this will normally show up asreplicas of the decay, shifted with respect to the peak of the principaldecay by a time equivalent to the distance between the optical surfacesinvolved (1 mm =8 ps at the speed of light) and with correspondinglysmaller amplitudes.

This drastic effect was first observed when using a very small areadetector in a time Correlated Single Photon Counting (TCSPC) set-up. Itwas found, after discriminating the photoluminescence signal againstscattered excitation and ambient light by polarisation selective andspectrally selective elements, that a further step, spatial filtering,was desirable to discriminate the main photoluminescence signal againstdelayed, multiply-reflected photons in order to minimise pulsebroadening (at low time resolution, slow decays) or the appearance ofreplicas (at high time resolution, fast decays) in the decay curve.

Spatial discrimination may be achieved in the PLS/ORM by refocussing thecollimated photoluminescence signal and defining a small aperture in theimage plane. The small size of the aperture, typically 5-20, micronsdiameter, makes it the system stop aperture. The spatial resolution ofthe spectrometer is then given by the field of view of the detector asdefined by the diameter of the stop aperture in the image plane and themagnification of the optics. Definition of the spatial resolution of thesystem through the spectrometer field of view rather than through theexcitation spot diameter has two important advantages. Firstly, thephotoluminescence signal is discriminated against multiple reflectionsor "ghost images". Signal photons crossing refractive optical surfacesat an angle with the surface normal will be scattered slightly out ofthe main beam path. In the prototype PLS it was found that spuriousreflections, especially from the surfaces of the polarising beamsplitter cubes, disappeared completely, once the stop aperture wascorrectly positioned in the image plane. This may conveniently beachieved by slightly tilting the planar optical components in theparallel beam path by a few degrees with respect to said optical axis,(i.e. the direction of signal propagation). Normally, the inevitablemisalignment of the beamsplitter cubes due to ordinary fabricationtolerances for the ORM mechanical housing will be sufficient to producethis desirable effect. Secondly, if the field of view of thespectrometer is much smaller than the excitation spot size, theexcitation density across the field of view will be almost homogeneous.This simplifies the physical interpretation of the resultingphotoluminescence decay in terms of a one-dimensional excitation densityprofile, i.e. a variation with depth only. Otherwise, for very smallexcitation spot sizes (several μm diameter) lateral diffusion ofcarriers out of the detector field of view would have to be taken intoaccount.

Ideally, if diffraction were negligible, the field of view of thespectrometer would be a well-defined circular area e.g. at the centre ofthe excitation spot, its radius being smaller than the excitation spotradius by at least several times the carrier diffusion length. Inpractice, diffraction will slightly smear out the abrupt spectrometerfield of view, which in the geometrical case and with no vignettingpresent, is a simple step function defined by the diameter of the stopaperture. The maximum excitation spot size will also normally be limitedby the power available from the excitation source, especially when usinga pulsed laser diode.

The separated out photoluminescence signal available at PBS3 can be usedto measure the instrumental response through a separate channel,simultaneously with the photoluminescence decay. This instrumentalresponse channel will be identical to the photoluminescence detectionchannel, except that the spectrally selective element, filter ormonochromator, is set to the excitation wavelength. Simultaneousacquisition of photoluminescence and (scattered) excitation signal inthe PLS would require duplication of the detector, time-convertor (TAC)and analog-to-digital converter (ADC) because of the very high datacollection rate of approximately 100 kHz (=0.2% of 50 MHz) to be handledunder optimum conditions by each channel.

The PLS can be used with pulsed laser diode excitation and single photonavalanche diode (SPAD) detection. This is a major advantage over anyother comparable system, whether it is a conventional TCSPC system witha micro-channel plate photomultiplier (MCP-PMT), a streak camera system(=time-domain methods) or a system with a phase sensitive detector(frequency domain method). These other systems always require a large,powerful and expensive laser source, usually a synchronously pumpedpicosecond dye laser system.

The use of only solid state components, i.e. a laser diode source and aSPAD detector, and the possibility of mechanically decoupling source anddetector from the microscope sample stage via optical fibre connectors,allow a simple, rugged and economically priced system to be provided inaccordance with the present invention. The low power densityrequirements of the PLS allow use of optical fibres forinterconnections, thus enhancing the modularity and flexibility of theoptical system, easing alingment and eliminating the need for an opticalbench/table.

The photoluminescence output detected by the SPAD then requires to beprocessed to allow measurement of the photoluminescent lifetime delaycharacteristics of the sample volume under inspection. The rapid decayof photoluminescence in GaAs substrates allows to take full advantage ofthe high repetition rate (up to 100 MHz) of commercially availablepulsed laser sources. Theoretically, this permits a data collection rateof 500 kcps (0.5% of 100 MHz) without risking distortion of the data dueto statistical pulse pile-up. In practice, the maximum data collectionrate is limited to around 100 kHz due to the finite conversion dead timeof even the fastest commercially available time-to amplitude converter(TAC), the Tennelec TC 863 (min. 1.6 μs dead time) and analog-to-digitalconverter/multichannel analyser combination (ADC/MCA9, the Silena Mod.7423/UHS ADC and Mod. 7328 MCA/buffer (3 μs fixed dead time). However,strong non-linearities in the response of all commercial TACs areobserved when the TAC is subject to low count rate random START signalsand the STOP input is triggered with a periodic signal at a frequencyabove 10 MHz. Obviously, NIM electronic devices such as the TAC aredesigned to operate with random signals up to 3×10⁷ cps in the STOP andSTART branch, but not with periodic signals as in TCSPC. For thisreason, cavity dumping has so far been considered essential in TCSPC.This has certainly not stimulated widespread application of the TCSPCtechnique, as the need for cavity dumping, usually available only inconnection with a synchronously pumped dye laser system, is clearly adisadvantage with respect to other TRPL methods capable of operating atthe high (76-100 MHz) mode-locking frequencies of a simple Nd:YAG or ionlaser.

So far, reduction of the high repetition rate signal to the much lowerPL signal rate by form of coincidence gating, has, although proposed inthe literature, not been widely used in TCSPC experiments. FIGS. 5A and5B show a new two stage anti-coincidence gating circuit. In more detailthe circuit makes use of four cascaded constant fraction discriminators(CFD1-CFD4), such as the commercially available Phillipps Scientific Mod714 QUAD CGD, and eliminates the need for any purpose built hardware.The circuit shown also includes delay means in the form of threevariable coaxial delay lines DEL2-DEL4) and one fixed delay coaxialcable (DEL1).

Three successive START pulses out of a continuous train of pulsescorresponding to the secondary beam laser excitation pulses are shown inFIG. 5A. In this particular example, the second START pulse isassociated with a single STOP pulse corresponding to a photoluminescencepulse induced by the laser excitation pulse to which the second STARTpulse corresponds, which is delayed with respect to the START pulse byδt. (In TCSPC, the statistical distribution of δt over a large number ofevents produces a histogram approximation of the photoluminescence decaycharacteristic). The START pulse is deliberately delayed relative to theSTOP pulse by DEL1. This allows a GATE pulse triggered by the STOP pulsein CFD1 to overlap the leading edge of the shaped second START pulseoutput from CFD2 present at the input of CFD3. The width of the GATEpulse is precisely adjusted to be Δt, the START pulse spacing.Synchronization of the reshaped second START output from CFD2 and theGATE pulse from CFD1 is achieved by varying the delay DEL2. The secondSTART pulse corresponding to the STOP pulse is eliminated in the outputfrom the anti-coincidence gated CFD3. The CFD3 output is used toanti-coincidence gate CFD4 so as then to produce a desired REDUCED STARTpulse train from the shaped, delayed and synchronized second output ofCFDI, which comprises only those START pulses for which a correspondingSTOP pulse has been received.

Any TCSPC system operating at high repetition rates (1 MHz) will benefitfrom the described circuit in three ways.

1 The periodic oscillations in the TAC response, due to the highrepetition rate periodic input, disappear.

2 The TAC can be operated in normal, i.e. non-inverted mode.

3 The useable time window Δt can be placed anywhere within the full TACrange.

As a result, the non-linearity of typically more than 2% at repetitionrates (STOP) above 10 MHz are reduced to the values specified forstandard operation, typically less than 0.8% (specified<1%).

In addition, in MCAs using Wilkinson type ADCs for converting the TACoutput, the overall conversion rate is strongly improved when the decaypeak is placed into a low channel. At very high repetition rates, e.g.100 MHZ, the width of the useable time window, Δt, is increasedsignificantly, e.g. by shifting the window away from the beginning ofthe TAC range, which is adversely affected by a fixed amount of deadtime and high non-linearity. At 100 MHz repetition rate, the useabletime window Δt is hence increased from less than 4 ns to almost fullpulse spacing -10 ns, an increase of more than 100%.

FIG. 5B shows generally the connections to a constant functiondiscriminator when used in a fast veto gating mode as in the case ofCFD3 and CFD4.

Finally FIGS. 6A and 6B show a typical photoluminescent decaycharacteristic curve for a Gallium Arsenide crystal obtained with a PLSof the invention such as shown in FIG. 1. As may be seen from thisresult, an instrumental response width of below 70 ps (FWHM) giving atime resolution of better than 10 ps and a spatial resolution of betterthan 3 μm allow the electrical characteristics of microscopic sampleregions or very small devices to be investigated.

A typical application of great commercial interest is the in-linequality control of laser diodes. At present, the quality of laser diodescan only be tested at the final stage, i.e after growing (10-15different steps), slicing, cleaving, packaging, bonding, encapsulatingthe device. It is known that the percentage of "scrap" devices isnormally very high, up to 90% of the total output. Although fataldefects are most likely to be introduced during the initial growthsteps, these defective devices are, at present, only revealed afteractually operating the fully processed devices. Hence, the efficiency ofthe manufacturing process could be greatly enhanced by identifyingfaulty devices (or precursors thereof) at an early stage of manufacturethrough analysis of the photoluminescence time response charactertisticsof the corresponding area of the GaAs crystal. The overall time requiredby a PLS-type system to test one device would be of the order of aslittle as several seconds.

The present invention also allows use of time-gated operation of a SPADdetector in electro-optic sampling as an alternative to using "slow"integrating photodiodes and boxcar signal averaging techniques forimproving signal-to-noise performance. The same basic TCSPC set-up couldbe used as in the PLS instrument, except that the signal to be measuredin electro-optic sampling is the change in intensity of the two planepolarization components after passing through the electroopticallyactive sample or probe device at the excitation wavelength of the pulsedexcitation source.

The possibility of improved signal-to-noise (s/n) better overall systemsensitivity and therefore shorter signal averaging time is due to thefact that a TCSPC detection system can be synchronised to themode-locking frequency of the exciting laser source. A single analyser(SCA) is used for monitoring the TAC output signals with amplitudescorresponding to events within a narrow time range around the peak. Thenumber of single photon events occurring within this narrow window areequivalent to the signal intensity measured with a slow, integratingphotodiode (PD). The counting of single photons over many excitationpulses at 76-100 MHz corresponds to improving the pD s/n ratio byaveraging over many lock-in cycles (several MHz).

The ratio of window width to repetition pulse spacing is a measure ofthe improvement in s/n of a fast single photon detector connected to aTAC and ADC/SCA relative to a slow integrating detector. The width ofthe SCA window will depend on the instrumental response width of theTCSPC system, typically 60 ps (FWHM) with a SPAD detector. A 100 ps SCAwindow, e.g. improved s/n of a 100 MHz electro-optic sampling systemwith TCSPC detection relative to conventional detection by a factor of100. before averaging.

Even signal averaging is faster in a TCSPC detection system because itoccurs at the high mode-locking frequency, while the fastestchoppers/lock-in amplifiers typically are limited to several MHz lock-infrequency and signal averaging is therefore much slower.

The external quantum efficiency of the current silicon SPAD at 1064 nmis very low, of the order of below 1%, such that little or no advantageis gained w.r.t. lock-in averaging systems based on slow integratingInGaAs or Ge PDs. However, application of Si-SPADs to the near infraredbeyond 1064 nm will be possible with devices having separate absorptionand avalanche multiplication regions, the latter consisting of e.g. anepitaxially grown strained layer Si_(1-x) Ge_(x) superlattice structureon top of the Si-SPAD.

The improved sensitivity of a TCSPC detection system in electro-opticsampling may allow pulsed diode lasers to be used as excitation sources,as in photoluminescence lifetime spectroscopy.

The system described here, based on TCSPC, may be used together. Theoptical routing module may moreover conveniently be suitably extended,using another set of polarising beamsplitter cubes, to allow operationof a single instrument as a PLS or an EOS system.

Time-correlated single photon counting (TCSPC) systems forfluorescence/luminescence lifetime measurements use standard NIM timingelectronics as in nuclear spectrocopy. The key component in any standardset-up is the time-to-amplitude converter (TAC)

TACs are designed for measuring the correlation between START and STOPsignals derived from statistical events with very high precision. Theintegral and differential non-linearity are typically better than 0.1%and 0.5% respectively at input data rates up to 30 MHz for randomsignals. Use of TACs with continuous repetition rate sources, which isthe case in fluorescence/luminescence lifetime experiments, however,results in decreased performance at much lower signal rates. Theintegral nonlinearity starts to increase significantly at below 10 MHzrepetition rate and may easily reach more than ±5% at 100 MHz.

The stop-rate-reducer (SRR) circuit used in the preferred embodiment ofthe present invention (FIG. 2a) uses four cascaded constant fractiondiscriminators (CFD1-4) with fast VETO gates and four coaxial delaylines (DEL1-4), in a special arrangement in order to achieve the desiredresult. QUAD CFD units with fast VETO, e.g. the PhiliPs Mod. 714 CFD andvariable coaxial delay lines are commercially available.

Three successive START pulses are shown in FIG. 2a. In this example, thesecond START pulse corresponds to the single STOP pulse, which isdelayed w.r.t. START by δt. The START pulse is delayed w.r.t STOP byDEL1. This allows the GATE pulse triggered by the STOP pulse in CFD1 tooverlap the leading edge of the shaped output from CFD2 present at theinput of CFD3 width of the GATE pulse is precisely adjusted to be Δt,the START pulse spacing. Synchronisation of shaped START output fromCFD2 and the GATE pulse from CFD1 is achieved by varying the delay DEL2.The START pulses corresponding to STOP pulses are now missing in theoutput form the gated CFD3. The CFD3 output is used to gate CFD4 whichthen produces the desired REDUCED START pulse train from shaped, delayedand synchronised second output of CFD1.

Thus in a further aspect the present invention provides gating devicesuitable for use in reducing a first very high speed periodic pulsetrain to a reduced pulse train containing only first pulsescorresponding to the pulses of a second pulse train induced byrespective ones of said first pulses with a variable time delay whichdevice comprises a first pulse divider having primary and secondary andsecondary outputs for primary and secondary first pulse trains, a secondpulse extender for extending said second pulses to provide an extendedsecond pulse train, a first fast gate means having an input forreceiving said primary first pulse train and a gating input forreceiving said extended second pulse train and disabling correspondingfirst pulses therein to provide a restricted output of first pulseswithout said corresponding first pulses, and a second fast gate meanshaving an input for receiving the secondary first pulse train and agating connection for receiving said restricted output of first pulsesand disabling first pulses therein corresponding to said restrictedoutput of first pulses so as to provide a reduced first pulse traincontaining only first pulses corresponding to said pulses of said secondpulse train.

Thus by using extended second pulses it is possible to disablecorresponding first pulses in the primary first pulse train despite thevariable time delay and non-synchronised relation between the first andsecond pulse trains. The resulting restricted first pulse train beingalready synchronised with the secondary first pulse train, the former isreadily employed selectively to disable all the first pulses thereofcorresponding to the restricted first pulse train thereby providing areduced first pulse train with only first pulses corresponding to thepulses of the second pulse train. With the arrangement of the presentinvention this can moreover be achieved at the very high repetitionrates used in laser systems with frequencies as high as 50 MHz to 100MHz.

Preferably the device includes second pulse divider means for providinga substantially non-extended second pulse train for comparison with saidreduced first pulse train. Advantageously at least one of said fastgating means comprises a constant fraction discriminator, and preferablyat least one of said pulse dividers and pulse extender comprises aconstant fraction discriminator, with, most conveniently a singleconstant fraction discriminator providing the second pulse divider andpulse extender.

It will of course be appreciated that various mofifications may be madeto the abovedescribed embodiments without departing from the scope ofthe present invention. Thus for example there may be used `inverted`microscope arrangements wherein the sample is inspected from belowrather than from above as illustrated, and indeed other arrangments withsimultaneous inspection from both above and below may also be used.

I claim:
 1. An optical routeing module device suitable for use in alight microscope for sample inspection simultaneously with a primarylight source and a secondary light source of different wavlength to saidprimary light source, which device comprises a housing mounting firstand second polarising beam splitter means along a primary light beampathway through said device and having secondary light beam inlet andoutlet means disposed opposite different, ones of said first and secondpolarising beam splitter means, said first and second polarising beamsplitter means each having a narrow predetermined operating wavelengthrange, defined between s-plane and p-plane transitional wavelengths,which substantially excludes the primary light source wavelength bandand is substantially above or below the wavelength band of said primarylight source and such that the wavelength band of each of the secondarylight source and a secondary light output from the sample responsive toincidence of said secondary light source on a said sample in use of thedevice is substantially below the s-plane transitional wavelength orabove the p-plane transitional wavelength, of a respective one of saidfirst and second polarising beam splitter means, respectively whereby,in use of the device, at least one polarising plane component of each ofthe secondary light source and said secondary light output is subjectedto a different one of transmission and reflection from that to which theprimary light source is subjected at each of the first and secondpolarising beam splitter means, said first and second polarising beamsplitter means further being formed and arranged for defining asecondary light beam pathway from said inlet to said outlet so that thesecondary light beam pathway is brought substantially into alignmentwith an outward leg of said primary beam pathway upstream of the sampleby said first polarising beam splitter means and is separated back outfrom a return leg of said primary light beam pathway downstream of thesample by said second polarising beam splitter means whereby in use ofthe device in a light microscope, the area of incidence of the secondarylight beam with the sample may be monitored via the primary light beam.2. A device according to claim 1 wherein at least one of said first andsecond polarising beam splitter means has an operating wavelength rangeof longer wavelength than said primary light beam source and defines aprimary light beam pathway section through itself by transmission.
 3. Adevice according to claim 2 wherein said at least one of said first andsecond polarising beam splitter means defines a secondary light beampathway for s-plane polarised light by reflection thereat.
 4. A deviceaccording to claim 1 wherein at least one of said first and secondpolarising beam splitter means has an operating wavelength range ofshorter wavelength than said primary light beam source and defines aprimary light beam pathway section at itself, by reflection.
 5. A deviceaccording to claim 4 wherein said at least one of said first and secondpolarising beam splitter means defines a secondary light beam pathwayfor p-plane polarised light by transmission thereat.
 6. A deviceaccording to claim 1 wherein the wavelength band of at least one of thesecondary light source and a secondary light output from the samplesresponsive to incidence of said secondary light source on a said samplein use of the device, is substantially within said predeterminedoperating wavelength range of a respective one of said first and secondpolarising beam splitter means.
 7. A device according to claim 1 whichincludes at least one half-wave plate means formed and arranged forchanging the plan of polarisation of a plane polarised secondary lightbeam approaching an associated polarising beam splitter means into acorresponding plane for a desired one of transmission through orreflection at said associated polarising beam splitter means.
 8. Adevice according to claim 1 which includes at least one plane reflectingmeans formed and arranged for guiding at least one of the primary andsecondary light beam pathways and reversing an image defined thereby. 9.A device according to claim 1 wherein the outward leg of said primarylight beam pathway passes through said first polarising beam splittermeans before said second polarising beam splitter means.
 10. A deviceaccording to claim 1 wherein the operating wavelength ranges of saidfirst and second polarising beam splitter means are at least partlyoffset relative to each other.
 11. A device according to claim 1 whereinsaid polarising beam splitter means are formed and arranged so that saidsecond polarising beam splitter means is crossed with respect to thefirst polarising beam splitter means or a combination of said firstpolarising beam splitter means with any polarising plane rotation meansprovided between said first and second polarising splitter means.
 12. Adevice according to claim 1 wherein the operating wavelength range ofeach of said first and second polarising beam splitter means is longerthan the primary light beam source wavelength and said first and secondpolarising beam splitter means are disposed along a substantiallyrectilinear primary light beam pathway with their principal planescrossed relative to each other whereby, in use, and s-plane polarisedsecondary light beam from said inlet is reflected at the firstpolarising beam splitter means substantially into alignment with theprimary light beam pathway and s-plane polarised secondary light outputbeam approaching said second polarising beam splitter means is reflectedthereat out of the primary light beam pathway towards said outlet. 13.An optical spectrometer device suitable for use in photoluminescenceinspection of microscopic areas of microstructures which devicecomprises a light microscope having infinity corrected optics with aprimary light beam pathway from a primary light source to a primary beamimage output means, via a sample stage which primary light beam pathwayhas a substantially common portion extending towards and away from saidsample stage, characterised in that there is provided an opticalrouteing module device according to claim 1 along said primary lightbeam pathway common portion, together with a secondary light beam sourceand a secondary light beam detector means coupled to respective ones ofsaid secondary light beam inlet and outlet means of said module, wherebythe position of incidence of said secondary light beam on said samplemay be precisely directed by monitoring of said primary beam imageoutput means.
 14. A device according to claim 13 wherein said secondarylight beam source is a laser source.
 15. A device according to claim 14wherein said laser source is a pulsed solid state laser source.
 16. Adevice according to claim 15 wherein said laser source is coupled tosaid optical routeing module device secondary light beam inlet means viaoptical fibre means and said secondary light beam outlet means iscoupled to said secondary light beam detector means by optical fibremeans.
 17. A device according to claim 13 wherein there is, coupled tothe secondary light beam outlet means, at least one of a solid statedetector, a fibre optic monochromator, and awavelength-division-demultiplexer.
 18. A device according to claim 13wherein the secondary light beam detector means comprises a singlephoton avalanche diode detector.
 19. A photoluminescent lifetimemicroscope spectrometer comprising a device according to claim 18wherein said secondary light beam source is a pulsed solid state lasersource and said single photon avalanche diode detector is connected toan anti-coincidence gating means coupled to the pulsed solid lasersource and formed and arranged for providing, in use, an outputcorresponding to laser source excitation pulses in which output alllaser source excitation pulses for which no secondary light output pulsefrom the sample is obtained, have been eliminated.
 20. A spectrometeraccording to claim 19 wherein said optical routeing module is formed andarranged so that said secondary light source beam is incident with saidsample at an angle in the region of the Brewster angle thereof.
 21. Aspectrometer according to claim 19 wherein said gating means comprisesfirst and second discriminator pulse processing means for receivingrespective ones of an excitation signal input comprising a series ofpulses corresponding to the laser source secondary light beam pulses anda detector signal output comprising a series of pulses corresponding tosecondary light output pulses induced by incidence of said laser sourcepulses with the sample in use of the device, said first pulse processingmeans being formed and arranged for providing first and second outputscomprising a first series of pulses corresponding to the laser sourcepulses substantially free of interference, said second pulse processingmeans being formed and arranged for providing a first output comprisinga second series of pulses corresponding to secondary light output pulsessubstantially free of interference and a second output comprising asecond series of gate pulses for respective ones of said secondary lightoutput pulses temporally extended to not longer than the period betweensuccessive said laser pulses; a third pulse processing means formed andarranged for receiving the second output of said first pulse processingmeans and said gate pulses and providing a modified output correspondingto said first series of pulses from which have been removed, by saidgate pulses, those pulses associated with said secondary light outputpulses thereby providing an anti-coincidence series of pulses; and afourth pulse processing means formed and arranged for receiving saidfirst output comprising a said first series of pulses and said modifiedoutput comprising said anti-coincidence series of pulses, and combiningthem so as to provide a reduced output comprising pulses correspondingto only those of said laser pulses for which a secondary light outputpulse has been received, said gating means further including pulse delaymeans formed and arranged for providing said first output of secondarylight output pulses in the same temporal relationship to said reducedoutput of pulses corresponding to corresponding laser pulses, as in theexcitation signal input and detector signal output whereby monitoring ofsaid temporal relation is substantially free of interference fromexcitation signals for which no detector signal output is received. 22.A device according to claim 1 wherein at least one of said first andsecond polarising beam splitter means is slightly bevelled and/or tiltedrelative to the primary light beam pathway thereby to reduce unwantedreflections when used with a small system aperture.
 23. A deviceaccording to claim 22 wherein is used a system stop aperture generallyin the image plane of from 5 to 50 um in diameter.
 24. A spectrometeraccording to claim 22 wherein said single photon detector and gatingmeans are connected to a time-amplitude converter means formed andarranged for converting the time delay between individual laser sourceexcitation pulses and their corresponding secondary light output pulses,into analogue signals.
 25. A method of simultaneous microsopic sampleinspection with primary and secondary light sources of differentwavelengths, which method comprises the steps of providing first andsecond polarising beam splitter means along a primary light beam pathwayof an optical microscope, said first and second polarising beam splittermeans each having a narrow predetermined operating wavelength rangewhich substantially includes a respective one of the secondary lightsource and a secondary light output from the sample responsive toincidence of said secondary light source on a said sample in use of thedevice, and excludes said primary light source; bringing a secondarylight beam pathway from said secondary light source substantially intoalignment with the primary light beam pathway at said first polarisingbeam splitter means and separating at least part of said secondary lightbeam pathway out of said primary light beam pathway at said secondpolarising beam splitter means, and detecting the separated outsecondary light beam output.